This invention relates to apparatus and method for calibrating transducers.
Transducers are used in a variety of applications to measure such things as position, pressure, or other physical or environmental characteristics. Position transducers, for example, detect positional movement in a variety of applications. One example position transducer is disclosed in U.S. Pat. No. 5,965,879 (the ""879 patent) and provides high sensitivity optical encoding. Such encoders have application in ultra-high precision, two-axis scanning mirror mechanisms. Of course, high resolution position transducers are extremely difficult to calibrate primarily because the test equipment must perform with greater precision than the transducer being tested.
Still, repeatable errors are induced within servo controlled systems that employ position feedback from inductor resolvers or optical encoders used in positional measurement. These errors are often removed or minimized using a calibration process involving the direct measurement of the position error at pre-determined locations within the motion range of the device. The error is measured statically, with the servo holding the mechanism at a fixed position for the duration of the measurement. As a result, error measurement requires very precise alignment of the measuring system to the mechanism and, in most cases, cannot be performed in-situ.
As described above, the direct measurement technique of prior calibration systems was a static operation. It required external devices to measure the error, which external devices had to have a higher accuracy than the system under calibration. For standard optical encoder systems, for example, devices such as theodolites were commonly used to provide high accuracy for calibration. Ultimately, devices which were costly, bulky, and complex were required to make direct measurement on the target system, which test devices were not only expensive but also difficult to perform in-situ.
In addition, the direct measurement technique was time consuming. Many measurements had to be made over the range of travel when errors had high spatial frequencies, as was the case with high resolution encoders. If the number of direct measurements was small to keep the test time low, interpolation of the data was required, which induced calibration error. Further, independent measurement devices required alignment by error-prone human operators.
In other prior calibration processes, repetitive measurement and control of repeatable errors in servo control systems have been employed to remove low frequency, disturbance induced errors. The repetitive system of calibration uses multiple measurements of the error to slowly correct the error.
The repetitive control technique had limited capability to correct error in measurements because the control bandwidth was very low. In such a system, sampling had to be kept at a low frequency in order to stay within appropriate Nyquist control characteristics (i.e., to remain stable). As a result, the repetitive calibration system did not accurately correct error signals outside of a low bandwidth. Further, an accurate measurement using servo error was not always possible because the amplitude and phase of the signal source appeared attenuated, amplified, or phase-shifted, depending upon frequency.
As one example, testing of the encoders in the HIRDLS system revealed a 0.6 arc seconds peak-to-peak cyclical error. To characterize and discover the source of the error, NASA brought in a new technology encoder to provide independent, high accuracy data. The test set up with both the operational encoder and the calibration encoder was used exhaustively over several months to characterize the nature of the cyclical error in the HIRDLS encoder. In contrast, an example embodiment of the present invention can perform the same test in a matter of hours without the need for the special calibration encoder. Thus, using the present invention, substantial savings can be realized in that a highly precise calibration encoder is not required in order to resolve repeatable errors. The calibration of a position transducer can be performed quickly, inexpensively, and in-situ.
The present invention provides calibration of repeatable errors in position transducers in a much preferable manner compared to either the direct measurement technique or the repetitive control technique of prior devices. In accordance with the present invention, direct measurements of error are taken in closed loop servo conditions. Unlike the direct measurement technique, no additional high precision calibration system is required, and tests can be performed quickly, inexpensively, and in-situ. Also, unlike the repetitive control technique, the present invention is not limited to calibration at only low control bandwidths.
In accordance with an example and preferred embodiment of the present invention, calibration of repeatable errors in position transducers is performed dynamically, in-situ under closed loop servo operation. The closed loop servo operation eliminates the man in the loop and allows for automatic, rapid, real-time measurement of the repeatable error.